1. Field of the Invention
This invention relates to the cross-protection of fowl against different serologic and immunologic types of Pasteurella multocida responsible for fowl cholera. In the United States alone, the cost of this disease to the turkey growers in the top 11 turkey-producing states (National Turkey Federation Symposium, University of Minnesota, 1970) amounted to at least $15 million. Although the acute form of the disease causes death in 1-5 days, a small number of infected birds may survive. Survivors are usually debilitated and are potential carriers of the disease.
2. Description of the Prior Art
The conventionl procedure for preparing fowl cholera bacterins is taught by Heddleston, Avian Diseases, Vol. VI, No. 3: 315-321 (1962). The Pasteurella multocida organisms are first isolated from infected fowl tissue with the aid of an agar medium. Selected colonies from the agar medium are then grown in tryptose broth medium, and the culture is then lyophilized and stored until use. Cells are grown from a lyophilized culture by the addition of a liquid medium such as tryptose broth, and incubated at 37.degree.. Routinely, the organism is subcultured on agar media to insure purity. To prepare a bacterin, the organism is then killed by addition of a sterilizing solution, such as formalin. The experiments shown in Heddleston, supra, illustrate that bacterins prepared by this procedure adequately protect vaccinated fowl against only the P. multocida strain homologous to that of the bacterin. Several experiments designed to test the benefit of enriched laboratory media on the development of better cross-immunizing bacterins have led to very little success. Other experiments by Heddleston illustrate that multivalent bacterins are sometimes effective against multiple strains of the organism, but the practicality of such is diminished by the added expense, the danger of immuno-suppression, and the difficulty in choosing the proper combination of serotypes. At the present time 16 different serotypes of Pasteurella multocida have been isolated, but the actual number is probably greater. Moreover, it is shown in Heddleston et al., Avian Diseases, Vol. IX, No. 3: 367-376 (1965) that bacterins containing only one strain of P. multocida gave better immunity in turkeys to homologous challenge than the bivalent and trivalent ones.
Cross-protection in turkeys has been achieved by Heddleston et al., Avian Diseases, Vol. 16, No. 3: 578-586 (1972) with fowl cholera bacterins prepared from turkey liver and blood tissue. Infected liver and heart blood was homogenized in a "Waring" blendor with formolized NaCl solution and centrifuged. The supernatant was used as the bacterin. Such a procedure, however, is economically unfeasible for immunizing large numbers of fowl. In this reference, Heddleston also demonstrates inducing cross-immunity in turkeys by administering a live avirulent mutant strain of P. multocida in the drinking water. However, the avirulent strain is reported to induce immunity for only a relatively short time, 3-6 weeks, and the live vaccine poses a health hazard for the more susceptible animal.
Some cross-protection was also observed by Heddleston et al., Avian Diseases, Vol. 18, No. 2: 213-219 (1974) by vaccinating turkeys with fowl cholera bacterins prepared from infected embryonating turkey eggs. As with the bacterins prepared from mature turkey tissue, this procedure is economically unfeasible.